If you want to really get into BM, then, beatmania 3rd Mix is pretty much the best mix.

If you're used to IIDX, and BM makes you confused and scared, then beatmania 6th + Core Remix has a couple of options that make it easier for IIDX players to pick up. It's a pretty "meh" mix if you haven't played the rest of the series though, as it's mostly fanservice.